Imagine batteries stretchy enough to flex inside clothing or under the skin. Unusually elastic batteries could one day help power flexible electronics worn on or implanted inside the body, researchers say.

"Some relatively simple but powerful ideas allow one to construct rechargeable, lithium-ion batteries that have the physical properties of a rubber band," says John Rogers, a materials scientist at the University of Illinois at Urbana-Champaign.

Applications may include robotic skin and wearable health monitors, he added.

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Increasingly, scientists are developing flexible electronics like video displays and solar panels that could make their way into clothing or even bodies. One limitation of these devices is the dearth of equally flexible batteries to power them or store any energy they generate.

Although past research has created bendable batteries, stretchability has proven a more challenging quality to engineer — past batteries could be stretched by no more than about 100 percent. In addition, no stretchable batteries have offered rechargeability with high storage capacity as one might expect of the lithium-ion technology now powering many smartphones, tablets, laptops and other mobile devices.

The new batteries Rogers and his colleagues have invented can be stretched by about 300 percent. Moreover, "our batteries are the first in this class to use rechargeable, lithium-ion technology," he says.

Altogether, the energy storage device is only about a half-millimeter thick, or roughly five times the average diameter of a human hair.

"We feel that the most important applications are in power supply for emerging classes of body-worn or implantable devices that are designed to laminate on the soft, curvy, dynamic surfaces of the human body," he adds.

Batteries generate electricity by electric current moving between a positively charged cathode and a negatively charged anode through an electrically conductive material called an electrolyte. The new battery uses flexible sheets of lithium cobalt dioxide cathode and lithium titanium oxide anode slurries linked via a gel electrolyte and sandwiched between soft layers of silicone rubber.

To collect electricity from the battery, the device incorporates an array of 100 disks of copper and aluminum connected by wires made of the same metals. These wires have wriggly, serpentine shapes, and just like corrugated paper, they can be pulled outward without ripping. In the team's current prototype, electricity is collected via a cable hooked up to a pair of pads at the edge of the battery. In future applications, they foresee the batteries being integrated directly into whatever devices they are powering.

Their advance has the added advantage of wireless inductive charging, he says — the kind seen with cordless power tools. This removes the need for connecting a plug into the device.

The capacity of these early prototypes is modest, at only a few percent of an AA battery. In addition, their lifetime of is limited by how well they can be isolated from environmental factors that can break them down, such as bodily fluids.

"This system is a first research prototype, developed in an academic lab. It is a starting point, and not an ending point, in a path to develop systems with real-world applicability," Rogers says.

Further optimization and refinement could boost the capacity of these prototypes by 100 times, he says. Future research can also focus on designing stretchable packaging materials that can protect them from water and oxygen.

"Although additional work will be required, we believe that improvements in capacity, lifetime and environmental isolation are possible," Rogers says.